A steel sheet for an outer sheet used for a side panel, a hood, or the like of an automobile has required panel rigidity and a dent resistance property (denting property). For improving the above denting property, it is effective to increase yield strength. On the other hand, for suppressing occurrence of surface strain to secure high surface accuracy when performing press forming, the yield strength is required to be decreased.
As a steel sheet that satisfies such two contradictory properties to achieve press formability and an increase in strength, a bake hardening (BH) steel sheet has been developed. The above BH steel sheet is a steel sheet whose yield strength increases by the baking finish treatment after press forming.
Here, the BH steel sheet will be explained in detail. FIG. 1A is a graph schematically showing a variation over time of yield strength of a conventional BH steel sheet. During the baking treatment after coating (while the steel sheet being heated to about 170° C. normally and maintained for several dozen minutes), C and N remaining in solution in the steel sheet diffuse to dislocation introduced at the time of press forming to firmly fix to the above dislocation, and thereby the yield strength increases. The above increased part of the yield strength is a bake hardening amount (BH amount), and the BH amount increases by increasing the content of solid solution C or the content of solid solution N in general.
However, such a hardening mechanism has the following problems. FIG. 1B is a graph schematically showing a variation over time of the yield strength of the conventional BH steel sheet in the case when the content of solid solution C or the content of solid solution N has been increased.
When the content of solid solution C or the content of solid solution N is increased in order to increase the BH amount, as shown in FIG. 1B, part of the dislocation is already fixed firmly by the solid solution C or the solid solution N before press forming (natural aging). Then, at the time of press forming, a wavy surface defect called stretcher strain occurs due to yield point elongation, and thereby a product property deteriorates remarkably. Furthermore, after baking finish, the solid solution C and the solid solution N precipitate as iron carbides and iron nitrides. Thereafter, as time passes, the carbides and nitrides grow, and further as the progress of coarsening continues, the yield strength decreases significantly.
It has been considered difficult to solve the above natural aging problem and to achieve a steel sheet satisfying both the natural aging resistance and the excellent bake hardenability, which has been a longstanding problem.
With regard to the above problem, there has been disclosed a method of adding Mo to thereby achieve bake hardenability and aging hardenability in Patent Document 1, Patent Document 2, and Patent Document 3.
Further, in Patent Document 4, there has been disclosed a method of controlling a rolling line load at the time of temper rolling and a steel sheet shape in the temper rolling to thereby prevent the occurrence of stretcher strain.